This project focuses on the mechanism and function of immune response (Ir) genes in the activation of thymus-derived T-lymphocytes using a secondary T-cell proliferation assay and the technology of T-lymphocyte cloning. With these techniques we have demonstrated that Ia molecules and antigen interact during T cell activation. In particular, for the antigen, moth cytochrome c, two parts of the molecule have been distinguished, one which interacts with the macrophage Ia molecule, the other with the T cell receptor.